Currently, most if not all cathode ray tubes (CRT) and liquid crystal displays (LCDs) are inspected visually to determine if all of the pixels in the display are functioning correctly. This is accomplished by displaying one or more patterns on the display while a person observes the pattern to spot defects. This method has several disadvantages. One disadvantage is that the person observing the patterns on the display must look closely and carefully at the entire screen in order to spot defects, which can be very time consuming, thus adding to the costs associated with the inspection. Another disadvantage of this method is that human beings are not always highly consistent in this type of inspection and, therefore, results can vary from day-to-day when performed by the same person and from person to person when performed by different people.
It is also known to inspect displays using high-resolution cameras. However, this type of inspection requires that the camera used to inspect the display have a higher resolution than the display being inspected. If the display has a low resolution, it is possible to use a high-resolution camera to inspect the display by feeding the output of the camera into a computer and having the computer analyze patterns on the display to determine whether or not defects exist in the display.
In most cases, a nine-to-one increase in the number of camera pixels to the number of display pixels is required in order to properly inspect the displays. This ratio requirement effectively limits the resolution of the display that can be tested and/or adds greatly to the cost of the camera used for the inspection. Furthermore, if a single camera is used to perform the inspection and the camera resolution is close to the display resolution, then the detection system can only detect large defects, i.e., defects covering more than a four-pixel area. Multiple cameras can be used in place of a single high-resolution camera to perform the inspection. However, using multiple cameras can greatly increase inspection time and cost of the test due to the added costs due to the added costs of multiple cameras and additional processing time.
Accordingly, a need exists for a method and apparatus for inspecting displays for defects which is capable of efficiently detecting defects, including small defects, and which overcomes the disadvantages of the existing inspection systems and methods.